The present invention relates to a temperature-dependent switch having a switching mechanism that has a movable contact part, which movable contact part cooperates with a stationary counter contact and is moved by a spring part to which the movable contact part is electrically conductively connected, the switching mechanism producing an electrically conductive connection between the stationary counter contact and a second counter contact in a temperature-dependent manner.
A switch of this type is known for example from DE 196 23 570 A1.
The known switch has a cup-like lower part which is closed by a flat upper part. A temperature-dependent switching mechanism is arranged inside the switch and carries a movable contact part, which cooperates with a stationary counter contact.
The switching mechanism comprises a snap-action spring disc, which carries the contact part and presses it against the stationary counter contact. Here, the snap-action spring disc is supported via its edge on the inner base of the lower part, which forms the second counter contact.
In this position, the two counter contacts are thus electrically conductively interconnected via the movable contact part and the snap-action spring disc.
The external connections are produced via the electrically conductive cover part, which is electrically conductively connected to the stationary counter contact, and via the electrically conductive lower part, on the inner base of which the snap-action spring disc is supported.
Above the snap-action spring disc, a bimetallic snap-action disc is arranged which lies loosely in the switching mechanism in its low-temperature position. In its high-temperature position, its centre presses the movable contact part away from the stationary counter contact, for which purpose it is supported via its edge on an insulating film, which is provided between the lower part and the upper part.
Whereas in the present case the spring part is a snap-action spring disc, against which a bimetallic snap-action disc works, it is also known to use merely a bimetal part as a spring part if the current can be conveyed directly through the bimetal part.
The known temperature-dependent switch is used to protect an electrical device against excessively high temperature. For this purpose, the supply current for the device to be protected is conveyed through the temperature-dependent switch, wherein the switch is coupled thermally to the device to be protected. At a response temperature predefined by the transition temperature of the bimetallic snap-action disc, the respective switching mechanism then opens the electric circuit in that the movable contact part is lifted from the stationary counter contact.
So that the switch does not close again once the device has cooled, it is further known, to provide in parallel to the temperature-dependent switching mechanism a self-holding resistor, preferably a PTC resistor, which, when the temperature-dependent switching mechanism is closed, is electrically short-circuited thereby. If the switching mechanism now opens, the self-holding resistor takes over some of the current flowing previously and in doing so heats up until it generates sufficient heat to keep the bimetallic snap-action disc at a temperature above the response temperature. This process is referred to as self-holding and prevents a temperature-dependent switch from closing again in an uncontrolled manner when the device to be protected cools down again.
Whereas in the case of temperature-dependent switches of this type an inherent heating of the spring part as a result of the flowing current is often undesirable, switches are also known in which a series resistor is additionally provided, which heats up in a defined manner as a result of the flowing current of the device to be protected. If the current flow is too high, this series resistor heats up to such an extent that the transition temperature of the bimetallic snap-action disc is reached. Besides the monitoring of the temperature of the device to be protected, the flowing current can thus also be monitored, and the switch then has a defined current dependency.
The spring part may also be a bimetal spring tongue, as is described in DE 198 16 807 A1. This bimetal spring tongue carries at its free end a movable contact part, which cooperates with a stationary counter contact. The stationary counter contact is electrically connected to a first external connection, wherein a second external connection is electrically connected to the fixed end of the bimetal spring tongue, which acts as a second counter contact.
The bimetal spring tongue, below its response temperature, closes the electric circuit between the two external connections by pressing the movable contact part against the stationary counter contact. In doing so, the bimetal spring tongue conveys the supply current of the electrical device to be protected.
If the temperature-dependent switch is to guide particularly high currents, a current transfer member in the form of a contact bridge or a contact plate is often used, which current transfer member is moved by the spring part and carries two contact parts which cooperate with two stationary counter contacts.
The supply current of the device to be protected thus flows from the first counter contact via the first contact part into the contact plate, through the contact plate to the second contact part and from there into the second counter contact. The spring part is therefore free from current. It is also known to use the spring part itself, that is to say for example a bimetallic snap-action disc or a snap-action spring disc working against a bimetal part, as a contact bridge.
Switches of this type have proven their value sufficiently in everyday use. If the switches do not open at the zero crossing of the AC supply voltage, an arc forms when the movable contact part is lifted from the stationary counter contact and the voltage drop across the switch reduces to the maintaining arc voltage. The voltage drop remains at this level until the applied AC supply voltage changes polarity, that is to say reaches its next zero crossing. The arc is then quenched and the switch is reliably opened.
The forming arcs lead to contact erosion and consequently in the long term to a change of the geometry of the switching areas of the movable contact part and stationary counter contact, which over time also leads to an impairment of the switching response.
In the event of uncontrolled flash-over in the interior of the switch, arcs even cause damage to the spring part. Arcs may also result in the switching areas sticking together, so to speak, such that the switch no longer opens or no longer opens quickly enough.
These problems even increase with the number of switching cycles, such that the switching response of the known switch is impaired over the course of time. Against this background, the life period, that is to say the number of permissible switching cycles of the known switch is limited, wherein the life period is also dependent on the switching power, that is to say the current intensity of the switched currents.
In particular towards the end of the life period of a temperature-dependent switch, the arcs in particular lead to such severe damage to the spring parts that the switch is damaged irreversibly.
Besides the contact erosion at the stationary counter contact and also the movable contact part, damage also occurs at the rim of spring discs, which spring disc carry the movable contact part and via their rim produce the electrical connection to the second counter contact. Over the course of the switching cycles, this leads to damage at the rim of the spring discs, whereby the life period is likewise limited.
On the whole, in the case of the known temperature-dependent switch, there is thus a link between the switching power and the maximum life period. The end of the life period of a switch is always accompanied by increasingly stronger arcs, which leads to contact erosion and sparks flying around, which damage the spring parts in the interior of switches of this type.
DE 977 187 A, in the case of a temperature-dependent switching mechanism that merely carries a bimetallic snap-action disc as a spring part, therefore proposes relieving this spring part of the current flow by connecting the movable contact part to the housing of the switch via a sun-gear-like metal spider which is supported internally on the switch. The current thus no longer flows through the bimetallic snap-action disc, but predominantly through the metal spider.
A similar approach is selected by AT 256 225 A, in which a copper branching is provided on the surface of the bimetallic snap-action disc remote from the stationary counter contact and connects the movable contact part to the housing.
In a development of the concepts from these two documents, DE 21 21 802 A proposes arranging, parallel to the bimetallic snap-action disc, a snap-action spring disc that produces the closing pressure of the switching mechanism and also carries the electric current. The bimetallic snap-action disc is thus relieved both mechanically and electrically, such that its life period is considerably extended.
Even with these switches, there is still the problem mentioned at the outset of the inevitably forming arcs that limit the life period of the known switch to a greater extent, the higher the switched current.